Solders are special composition metals (known as alloys) that, when in the presence of flux, melt at relatively low temperatures (120-450° C.). The most commonly used solders contain tin and lead as base components. Many alloy variations exist that include two or more of the following metallic elements: tin (Sn), lead (Pb), silver (Ag), bismuth (Bi), antimony (Sb) and copper (Cu). Solder works by melting when it is heated, and bonding (wetting) to metallic surfaces. The solder forms a permanent intermetallic bond between the metals joined, essentially acting like a metal “glue.” In addition to providing a bonding function, solder joints also provide an electrical connection between soldered components and a heat transfer path. Solders are available in many forms including paste, wire, bar, ribbon, preforms and ingots.
Generally, metal surfaces have a thin film of oxidation or passivation caused by normal environmental exposure to air and oxygen that acts as a barrier during the soldering process. Accordingly, a chemical product (usually rosin-based) known as “flux” is used to prepare the metal surfaces for soldering by cleaning off oxides, passivation and other contamination. Flux also reduces the surface tension of the solder alloy to promote wetting out over exposed solderable surfaces beyond the initial deposit location. During the preheating stage, the flux is working and the alloy is approaching its melting point. After the solder becomes completely molten, heat is removed to allow re-solidification of the alloy in its new position.
There are four basic flux types to choose from that provide a wide variety of capabilities. No-Clean (NC) flux consists of rosin, solvent, and a small amount of activator. NC flux has low activity and is suited to easily solderable surfaces. NC residue is clear, hard, non-corrosive, non-conductive, and designed to be left on the assembly. Residue may be removed with an appropriate solvent if so desired. Rosin Mildly Activated (RMA) flux consists of rosin, solvent, and a small amount of activator. Most RMA flux is fairly low in activity and best suited to easily solderable surfaces. RMA flux residue is clear, soft, non-corrosive, and non-conductive. Cleaning is optional. Residue may be removed with an appropriate solvent if desired. Rosin activated (RA) flux consists of rosin, solvent, and aggressive activators. RA flux has higher activity than RMA for moderately oxidized surfaces. RA flux residue is corrosive and should be removed as soon as possible after reflow to prevent damage to the assembly. Maximum safe time before cleaning is product dependent. Residue may be removed with an appropriate solvent. Water Soluble (WS) flux consists of organic acids, thixotrope, and solvent. WS flux comes in a range of activity levels for soldering to even the most difficult surfaces. WS flux residue is corrosive and should be removed as soon as possible after reflow to avoid damage to the assembly. The maximum safe time before cleaning is generally product dependent. Typically, residue may be removed with 60° C. (140° F.) water and 40 psi pressure.
A problem with the foregoing conventional soldering approaches is that the use of flux is intolerable under some manufacturing processes. For example, all fluxes, including even NC and RMA fluxes, leave residues containing contaminants that are unsuitable for manufacturing certain types of semiconductor-based assemblies and optical communication equipment. These situations call for the use of fluxless solder assembly techniques.